From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Wed, 30 May 2001 12:23:28 -0400
There are "Rules of Thumb" for depth to span ratios for trusses that have
been shown to produce "economical" trusses. Basically, all of them relate to
the moment diagram for a uniformly loaded member. Usage might require
deeper or shallower depths. (Note joist depth/span ratio.) These are from
memory right now as I haven't dragged out my references:
Pitched: depth = 1/8 span
Parallel: depth = 1/10 span
Bowstring: depth = 1/8 span
Continuous: depth = 1/12 to 1/15 span
Joists usually have a depth of 1/25 to 1/27 span.
Of these, the relationship to the moment diagram can be seen most clearly
with the bowstring truss. The force in the top chord and the force in the
bottom chord remains essentially constant from the middle of the span to
the end of the span resulting in uniform sized chord members. Web members
carry very little force.
Here in the desert southwest, roof mounted air condidioners and coolers are
common and allowances should be made for something that would be mounted on
the roof. The large (60" X 60") evaporative coolers, with soaked pads and
water in the reservoir, will weigh upwards of 1,100 pounds.
A. Roger Turk, P.E.(Structural)
Greg Effland wrote:
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First I would like to more correctly state something I had recommended to
you about stitch welding the double angles together... I think someone else
pointed this out correctly but when welded together the kl/r between welds
of each member (angle) should not exceed 3/4 of the kl/r of the composite
member (as per AISC ASD 9th Ed. Section E4 Paragraph 5)
I think applying the 5 kips (if desired) should be done at bottom panel
points. I think the check with the loads at the top panels might be overly
conservative. If it were to be used as a hoisting point most people would
go to the closest point... besides the roof panel may not allow someone to
go above the top chord.
Collateral Load is essentially Dead Load. It should be included with dead
load in any "Gravity" load combinations and NOT included with wind uplift
load combinations. The theory behind this approach is that the "collateral"
load may not be attached to every supporting member, it might be connected
to every other one or something like that. If assumed it may not connect to
each supporting member then it is conservatively placed with gravity
combinations and conservatively NOT placed with uplift combinations.
Quick Moment/Depth (M/D) check... This can be done at any length along the
truss. Typically I would start at the highest moment. Since most trusses
are simple span the highest moment occurs in the middle of the span. For
open web frames (I think this is the case you are asking about) there might
be a fixed connection at the sidewall columns. This will alter the moment
diagram and this should be taken into account. What I would do , and keep
in mind this is not exact and only provides a starting point, is calculate
the maximum moment and divide this number by the assumed total depth of the
truss at that point.
20 ft trib width
40 psf roof load
Max. Moment (Assume 110 ft simple span) = (wl^2)/8 = 14,520 kip-in
Assumed depth = 4 ft = 48 inches
M/d = 302.5 kips
Assuming 0.6*Fy*Area (Fy=50 ksi, High Strength Steel) *** Might consider
using 0.66*Fy or 0.75*Fy to make sure you don't miss a lighter case that may
0.6*Fy= 30 ksi
Area(Required)= (M/d)/(0.6*Fy) = 10.08 Inches
Use a chord with Area >= 10.08 inches (Usually pick a selection that is
readily available, refer to Modern Steel Construction, they annually publish
charts showing which sections are more available then others... or consult
your standard steel supplier)
This gives you a ROUGH area to start with. A chord with the area less than
calculated above would likely not work. If .66Fy was used then the
possibilty of a section with less area working decreases. If used .75Fy was
used then I would think only the rarest cases (if any) would work with less
area than calculated above.
Hope this helps,
Greg Effland, P.E.
From: Juan José Treff De la Mora [mailto:jjtreff(--nospam--at)hotmail.com]
Sent: Tuesday, May 29, 2001 5:47 PM
Subject: MORE ABOUT TRUSSES...
Recently I posted a question on roof trusses. Many people responded me with
very good advise but I have some new doubts and would like to know your
Mark advised me to include 5 extra kips at the worst joints or nodes due to
lifting. Did I understand right? How would you pick the "worst ponits"? Do
you do it only in the lower chord nodes?
Then John advised me to add 10 psf for ceilling weight. And Greg told me to
add from 3 to 10 psf for collateral loads.
Do I add this loads to the Dead Load condition, to the Live Load condition
or as a new Collateral load condition?
For the initial truss depth, Greg adviced me to do a quick moment/depth calc
and check against ASD tension check. Coul you be a little bit more explicit?
Thank you all for your kind and valuable help.
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